Digital Signal Processing Reference
In-Depth Information
21.3.4 Echoes, Multipath Reception
Echoes, i.e. multipath reception, lead to frequency-selective fading. There
is interference in individual carrier ranges but the information lost as a re-
sult can be restored again due to the interleaving across the frequency and
the large amount of error protection (Reed Solomon and convolutional
coding) provided in DVB-T. Of course, COFDM (Coded Orthogonal Fre-
quency Division Multiplex) was developed precisely for this purpose,
namely to cope with the effects of multipath reception in terrestrial trans-
mission.
21.3.5 Doppler Effect
In mobile reception, a frequency shift occurs over the entire DVB-T spec-
trum due to the Doppler effect. By itself, the Doppler effect does not pre-
sent a problem in DVB-T transmission because a shift of a few hundred
Hertz at motor vehicle speeds can be handled easily. It is when Doppler ef-
fect and multipath reception are combined that the spectrum becomes
smeared. Echoes moving towards the receiver will shift the spectrum into a
different direction from those moving away from the receiver and, as a re-
sult, the signal/noise ratio in the channel deteriorates.
21.3.6 I/Q Errors of the Modulator
The focus of this discussion will now shift to the I/Q errors of the DVB-T
modulator, the effects of which differ from those in DVB-C.
The COFDM symbol is produced by means of the mapper, the real parts
and imaginary parts of all subcarriers being set in the frequency domain
before the IFFT (Inverse Fast Fourier Transform). Each carrier is inde-
pendently QAM modulated (QPSK, 16QAM, 64QAM) in accordance with
the information to be transmitted. The spectrum has no symmetries or
centro-symmetries and thus is not conjugate with respect to the IFFT band
center.
According to system theory, therefore, a complex time-domain signal
must be produced after the IFFT. Considering then the real time-domain
signal re(t) and the imaginary time-domain signal im(t) carrier by carrier, it
is found that for each carrier, re(t) has exactly the same amplitude as im(t)
and that im(t) is always shifted by exactly 90 degrees in phase with respect
to re(t). All re(t) superimposed in time are fed into the I branch of the
complex I/Q mixer and all im(t) superimposed in time are fed into the Q
branch. The I mixer is fed with 90 degrees carrier phase and the Q mixer is
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